During the past years, with the increase in price of fossil fuels, the interest in developing new wells has dramatically increased. When drilling a well, for example, in oil and gas exploration applications, safety devices are put in place to prevent injury to personnel and damage to environment and/or equipment resulting from unexpected events associated with the drilling activities. Thus, well control is an important aspect of oil and gas exploration.
Drilling wells in oil and gas exploration involves penetrating a variety of subsurface geologic structures, or “layers.” Occasionally, a wellbore will penetrate a layer having a formation pressure substantially higher than the pressure maintained in the wellbore. When this occurs, the well is said to have “taken a kick.” The pressure increase associated with the kick is generally produced by an influx of formation fluids (which may be a liquid, a gas, or a combination thereof) into the wellbore. The relatively high pressure kick tends to propagate from a point of entry in the wellbore uphole (from a high pressure region to a low pressure region). If the kick is allowed to reach the surface, drilling fluid, well tools, and other drilling structures may be blown out of the wellbore and the integrity of the well may be destroyed with grave consequences for the environment (e.g., uncontrolled oil spills undersea). These “blowouts” may also result in catastrophic destruction of the drilling equipment (including, for example, the drilling rig) and substantial injury or death of rig personnel.
Because of the risk of blowouts, blowout preventers (“BOPs”) are typically installed at the surface or on the sea floor in deep water drilling arrangements to effectively seal a wellbore until active measures can be taken to control the kick. BOPs may be activated so that kicks are adequately controlled and “circulated out” of the system. There are several types of BOPs, one common type of which is an annular blowout preventer.
Annular BOPs typically includes annular, elastomeric “packing units” that may be activated to encapsulate drillpipe and well tools to completely seal about a wellbore. In situations where no drillpipe or well tools are within the bore of the packing unit, the packing unit can be compressed to such an extent that the bore is entirely closed, acting as a valve on the wellbore. Typically, packing units are used in the case of sealing about a drillpipe, in which the packing unit can be quickly compressed, either manually or by machine, to effect a seal about the pipe to prevent a well from blowing out.
An example of an annular BOP having a packing unit is disclosed in U.S. Pat. No. 2,609,836, (“Knox”) and incorporated herein by reference in its entirety, the assignee of the present invention. The packing unit includes a plurality of metal inserts embedded in an elastomeric body. Upon compression of the packing unit about a drillpipe, or upon itself, to seal against the wellbore pressure, the elastomeric body is squeezed radially inward, causing the metal inserts to move radially inward as well.
FIG. 1 is an example of a background art annular BOP 101 including a housing 102. The annular BOP 101 has a bore 120 extending therethrough and is disposed about a longitudinal axis 103. A packing unit 105 is disposed within the annular BOP 101 about the longitudinal axis 103. The packing unit 105 includes an elastomeric annular body 107. The packing unit 105 includes a bore 111 concentric with the bore 120 of the BOP 101.
The annular BOP 101 is actuated by fluid pumped into opening 113 of a piston chamber 112. The fluid applies pressure to a piston 117, which moves the piston 117 upward. As the piston 117 moves upward, the piston 117 translates force to the packing unit 105 through a wedge face 118. The force translated to the packing unit 105 from the wedge face 118 is directed upward toward a removable head 119 of the annular BOP 101, and inward toward the longitudinal axis 103 of the annular BOP 101. Because the packing unit 105 is retained against the removable head 119 of the annular BOP 101, the packing unit 105 does not displace upward from the force translated to the packing unit 105 from the piston 117. However, the packing unit 105 does displace inward from the translated force, which compresses the packing unit 105 toward the longitudinal axis 103 of the annular BOP 101. In the event a drill pipe 130 is located along the longitudinal axis 103, with sufficient radial compression, the packing unit 105 will seal about the drill pipe into a “closed position.” The open position is shown in FIG. 2 while the closed position is shown in FIG. 3. In the event a drill pipe is not present, the packing unit 105, with sufficient radial compression, will completely seal the bore 111.
An example of the packing unit 105 used in an annular BOP 101 is shown in FIG. 4. As before, the packing unit 105 includes an elastomeric annular body 107 and may include a plurality of metallic inserts 109. The metallic inserts 109 may be distributed at equal radial distances from each other in the elastomeric annular body 107 of the packing unit 105. The packing unit 105 includes the bore 111.
The traditional packing units use for the elastomeric annular body nitrile rubber (NBR), which is the work horse in BOP applications because of its good physical properties and oil resistance. However, NBR exhibits accelerated chemical degradation when exposed to zinc bromide (ZnBr.sub.2) fluid, which is a component of the “mud” used in the wells for various purposes. An alternative to NBR is the Fluorocarbon Elastomer (FKM), which has better resistance to chemicals including ZnBr.sub.2. However, FKM is not mechanically strong as the NBR and it is also expensive.
Another approach to improve the chemical degradation is to surface fluorinate NBR molded BOP parts. U.S. Pat. Nos. 5,214,102 and 5,274,049 (the contents of which are included by reference in their entirety herein) describe fluorination of molded elastomeric articles for reducing static and dynamic coefficients of friction and improving the wear life of the articles. Specifically, these two documents considered fluorinating elastomers as Kraton, Hytrel and other thermoplastic rubbers (a material that have both elastomeric and thermoplastic properties). Along the same lines, a paper entitled “Enhancement of the Chemical Resistance of Nitrile Rubber by Direct Fluorination” published in the Journal of Applied Polymer Science, Vol. 89, pages 971-979 (2003) (the entire content of which is incorporated in its entirety herein), investigates the addition of a fluorination layer on a nitrile rubber to prevent chemicals from penetrating inside the rubber and observing a retardation of loss of the mechanical properties.
Hydrogen sulfide (H2S) in sour crude oils is a great challenge to elastomer blow out preventer (BOP) packers and seals. Many such packers and seals include nitrile elastomer. H2S, however, degrades typical nitrile elastomer materials. Some known fluoroelastomers have better H2S compatibility, but are much more expensive and mostly are not suitable to BOP applications based on existing compound technology.
However, the NBR with the surface fluorination is still not good enough for the present applications, especially that it was observed that cracks appear in the elastomeric material when the fluorination is performed. Accordingly, it would be desirable to provide systems and methods that avoid the afore-described problems and drawbacks.